A lithographic apparatus is a machine that applies a desired pattern onto a workpiece or part of a workpiece. A lithographic apparatus may be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. A conventional lithography apparatus includes, among other things, an illumination system to produce a uniform intensity distribution of a received laser beam. It is desirable that the resulting illumination be as uniform as possible and that any uniformity errors be kept as small as possible. Illumination uniformity influences an ability of the illumination system to produce uniform line widths across an entire exposure field. Illumination uniformity errors significantly impact quality of devices produced by the lithographic apparatus.
The conventional lithographic apparatus has a conventional filter having a pattern of opaque dots of modulated density that corrects exposure field non-uniformity. The dots are positioned out of focus to avoid projection onto the exposure field. Unfortunately, the dots diffract incident light and limit achievable spatial frequencies. The diffraction of the incident light distorts illumination modes, particularly for a conventional lithographic apparatus having small-coherence illuminations and small numerical apertures. Further, for a lithographic system with a large demagnification, such as an optical maskless lithography system, the conventional filter also requires submicron-sized dots, which makes the conventional filter expensive.
The conventional filter is formed on a plano-parallel substrate. Thus, if a projected image is to be magnified or demagnified in addition to being filtered by the conventional filter, then the conventional lithographic apparatus requires additional optical elements to adjust the magnification. This requirement unnecessarily adds complexity and excessive cost to the conventional lithographic apparatus.
Therefore, what is needed is an apparatus and method that overcomes the above shortcomings.